Back to EveryPatent.com
| United States Patent |
5,787,130
|
|
Kotzin
, et al.
|
July 28, 1998
|
Method and apparatus for canceling interference in a spread-spectrum
communication system
Abstract
A technique for decoding and removing a single subscriber's signal from a
composite signal in a DS-CDMA system is provided. A particular
subscriber's signal is removed from the composite signal by despreading
the composite signal (120) to form a multiplicity of despread QPSK signals
representative of multiple subscribers. Next, multipath characteristics
are determined (303) for each of the multiple subscribers. Utilizing
multipath characteristics (204), the multiple subscribers' signals are
determined by combining multipath components of each signal (305). Next,
data related to a particular subscriber is determined (307) and the
subscriber's signal is "reconstructed" to contain multipath
characteristics (204) as originally received (309). Finally, the
reconstructed signal is output to a summing node (128) where it is
combined with the delayed composite signal (120).
| Inventors:
|
Kotzin; Michael D. (Buffalo Grove, IL);
Meidan; Reuven (Ramat Hasharon, IL)
|
| Assignee:
|
Motorola Inc. (Schaumburg, IL)
|
| Appl. No.:
|
763160 |
| Filed:
|
December 10, 1996 |
| Current U.S. Class: |
375/346; 370/342; 370/441; 375/148; 375/150; 455/296 |
| Intern'l Class: |
H04B 001/10; H03D 001/04 |
| Field of Search: |
375/200,206,346,348
327/310
348/607
455/296
370/342,441
|
References Cited
U.S. Patent Documents
| 5235612 | Aug., 1993 | Stilwell et al. | 375/200.
|
| 5418814 | May., 1995 | Hulbert | 375/346.
|
| 5553062 | Sep., 1996 | Schilling et al. | 375/346.
|
| 5687162 | Nov., 1997 | Yoshida et al. | 375/346.
|
Primary Examiner: Ghebretinsae; Temesghen
Attorney, Agent or Firm: Haas; Kenneth A.
Claims
What we claim is:
1. A method of canceling interference in a spread-spectrum communication
system, the method comprising the steps of:
receiving a first spread-spectrum signal having a plurality of subscribers
modulated thereon;
identifying multipath characteristics for the plurality of subscribers;
utilizing the multipath characteristics to construct a second
spread-spectrum signal representative of the first spread-spectrum signal
free from signals caused by multipath scattering;
extracting information from the second spread-spectrum signal
representative of an individual subscriber to produce extracted
information;
utilizing the extracted information and the multipath characteristics to
produce a reconstructed signal representative of the individual
subscriber, wherein the reconstructed signal representative of the
individual subscriber includes multipath scattering components; and
inverse summing the reconstructed signal representative of the individual
subscriber with the first spread-spectrum signal to produce a third
spread-spectrum signal substantially free of any interference contributed
by the individual subscriber.
2. The method of claim 1 wherein the step of extracting information from
the second spread-spectrum signal representative of the individual
subscriber comprises the steps of:
determining from the first spread-spectrum signal having a plurality of
subscribers modulated thereon, a subscriber having a most reliable signal;
and
extracting information from the second spread-spectrum signal
representative of the subscriber having the most reliable signal to
produce extracted information.
3. The method of claim 2 wherein the step of extracting information from
the second spread-spectrum signal representative of the subscriber having
the most reliable signal comprises extracting information from the second
spread-spectrum signal representative of the subscriber having a highest
ratio of energy per information-bit to noise-spectral density (E.sub.b
/N.sub.0).
4. The method of claim 1 wherein the step of receiving the first
spread-spectrum signal comprises the step of receiving the first
spread-spectrum signal having a multiplicity of frequency and time
overlapping coded (spread) signals each of which has undergone multipath
scattering.
5. The method of claim 1 wherein the step of identifying multipath
characteristics comprises the step of identifying characteristics from a
group consisting of time delays, respective amplitudes, and phases between
correlation peaks.
6. A method of canceling interference in a spread-spectrum communication
system, the method comprising the steps of:
receiving a composite signal having a multiplicity of frequency and time
overlapping coded (spread) signals each of which has undergone multipath
scattering;
despreading the composite signal to form a multiplicity of despread signals
representative of a plurality of subscriber transmitted signals;
identifying multipath characteristics for the plurality of subscriber
transmitted signals;
utilizing the multipath characteristics to combine multipath components of
the plurality of subscriber transmitted signals to produce a plurality of
corrected subscriber transmitted signals;
extracting information related to a first signal from the corrected
subscriber transmitted signals;
reconstructing, as originally received, the first signal, wherein the
reconstructed first signal contains multipath scattering components of the
first signal as originally transmitted; and
inverse summing the reconstructed first signal with composite signal to
produce a third spread-spectrum signal substantially free of any
interference contributed by the first signal.
7. The method of claim 6 wherein the step of extracting information related
to the first signal comprises the steps of:
determining from the multiplicity of despread signals representative of the
plurality subscriber transmitted signals, a subscriber having a most
reliable signal; and
extracting information related to a first signal from the corrected
subscriber transmitted signals representative of the subscriber having the
most reliable signal.
8. The method of claim 6 wherein the step of identifying multipath
characteristics comprises the step of identifying characteristics from a
group consisting of time delays, respective amplitudes, and phases between
correlation peaks.
9. An apparatus for canceling interference in a spread-spectrum
communication system, the apparatus comprising:
a despreader having as an input a composite signal and outputting a
plurality of despread signals;
a multipath identifier having as an input the plurality of despread
signals, and outputting multipath characteristics of the plurality of
despread signals;
a RAKE finger combiner having as an input the plurality of despread signals
and the multipath characteristics of the plurality of despread signals,
and outputting a first signal which is a representative of the composite
signal without multipath scattering components;
a data decoder having as an input, the first signal and outputting
information related to an individual signal;
a signal reconstructor having as inputs, information related to the
individual signal and the multipath components, and outputting the
individual signal as originally received, wherein the individual signal as
originally received contains multipath scattering components; and
an inverse summer having as inputs the individual signal as originally
received and the composite signal and outputting the composite signal
substantially free of any interference contributed by the individual
signal.
10. The apparatus of claim 9 further comprising an ordering generator
having as an input the composite signal, and outputting a most reliable
signal.
11. The apparatus of claim 10 wherein the most reliable signal comprises a
signal with the highest ratio of energy per information-bit to
noise-spectral density (E.sub.b /N.sub.0).
Description
FIELD OF THE INVENTION
The present invention generally relates to canceling interference in
signals in a communication system, and more particularly to a method and
apparatus for canceling interference in signals having undergone multipath
scattering.
BACKGROUND OF THE INVENTION
In a communication system such as a direct sequence spread-spectrum code
division multiple access (DS-CDMA) system, a received signal at a base
station comprises a multiplicity of frequency and time overlapping coded
signals from individual subscribers. Each of these signals is transmitted
simultaneously at the same radio frequency (RF) and is distinguishable
only by its specific encoding. In other words, the uplink signal received
at a base-station receiver is a composite signal of each transmitted
signal and an individual subscriber's signal is distinguishable only after
decoding.
In conventional DS-CDMA systems, the receiver decodes each subscriber
separately by applying each respective subscribers' code to the composite
received signal. Each individual subscriber's signal is thereby "despread"
from the composite received signal. Due to the nature of the family of
codes utilized, the other subscriber's signals remain in a spreaded form
and act only to degrade the recovered signal as uncorrelated interference.
This allows the decoding of subscriber data bits for a particular
subscriber.
Prior art techniques of interference cancellation are known to reduce even
the uncorrelated interference. This permits an increase in the sensitivity
and or capacity of the multi-subscriber system. The most common technique
is to synthesize a replica of a particular subscriber's received signal,
after it has been properly decoded, and utilize the synthesized replica to
cancel interference (by subtraction) in the received signal. Such a
prior-art method of interference cancellation is described in U.S. Patent
"Method and Apparatus for Canceling Spread-Spectrum Noise" by Stilwell,
et. al., (U.S. Pat. No. 5,235,612) assigned to the assignee of the present
invention, and incorporated herein by reference. By utilizing such
prior-art techniques it is possible to effectively eliminate a
subscriber's signal from the composite received signal such that the
decoding of subsequent subscriber's signals is accomplished with greater
accuracy.
In a land mobile environment, received signals from subscribers undergo
multipath scattering. In other words, a signal transmitted by a subscriber
undergoes multiple reflections before it is received at a receiver, and
these reflections cause "echoes" of the transmitted signal to be
simultaneously received by the receiver. These echoes are generally of
different amplitudes and different time delays, and therefore cause a
signal received from each subscriber to actually consists of a
multiplicity of signals (the actual signal and its echoes), each having a
different amplitude and time delay. Such multi-path scattering causes
significant contribution to the interference at the receiver.
Because of multipath scattering, prior-art interference cancellation
techniques are deficient in the synthesis of the replica of a particular
subscriber's received signal since the synthesis of the replica does not
take into consideration the multipath nature (i.e., the echoes) of the
received signal. Therefore, a need exists for improved interference
cancellation which accounts for multipath scattering of the received
signals.
DESCRIPTION OF THE DRAWINGS
FIG. 1 generally depicts, in block diagram form, a receiver unit which may
beneficially implement interference cancellation in accordance with the
invention.
FIG. 2 generally depicts, in block diagram form, a RAKE based signal
generator of FIG. 1 in accordance with the invention.
FIG. 3 is a flow chart illustrating operation of a signal cancellation unit
of FIG. 1 in accordance with the preferred embodiment of the present
invention.
FIG. 4 generally depicts, in block diagram form, a receiver unit which may
beneficially implement interference cancellation in accordance with an
alternate embodiment of the present invention.
FIG. 5 generally depicts, in block diagram form, a most reliable signal
selector of FIG. 4 in accordance with an alternate embodiment of the
present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Stated generally, a technique for decoding and removing a single
subscriber's signal from a composite signal in a DS-CDMA system is
provided. A particular subscriber's signal is removed from the composite
signal by despreading the composite signal to form a multiplicity of
despread QPSK signals representative of multiple subscribers. Next,
multipath characteristics are determined for each of the multiple
subscribers. Utilizing multipath characteristics, the multiple
subscribers' signals are determined by combining multipath components of
each signal. Next, data related to a particular subscriber is determined
and the subscriber's signal is "reconstructed" to contain multipath
characteristics as originally received. Finally, the reconstructed signal
is output to a summing node where it is combined with the delayed
composite signal.
The present invention encompasses a method of canceling interference in a
spread-spectrum communication system. The method comprises the steps of
receiving a first spread-spectrum signal having a plurality of subscribers
modulated thereon and identifying multipath characteristics for the
plurality of subscribers. Next, multipath characteristics are utilized to
construct a second spread-spectrum signal representative of the first
spread-spectrum signal free from signals caused by multipath scattering.
Information is then extracted from the second spread-spectrum signal
representative of an individual subscriber to produce extracted
information, and this information is utilized to produce a reconstructed
signal representative of the individual subscriber, where the
reconstructed signal representative of the individual subscriber includes
multipath scattering components. Finally, the reconstructed signal is
inverse summed with the first spread-spectrum signal to produce a third
spread-spectrum signal substantially free of any interference contributed
by the individual subscriber.
An alternate embodiment of the present invention encompasses a method of
canceling interference in a spread-spectrum communication system. The
method comprises the steps of receiving a composite signal having a
multiplicity of frequency and time overlapping coded (spread) signals each
of which has undergone multipath scattering and despreading the composite
signal to form a multiplicity of despread signals representative of a
plurality subscriber transmitted signals. Next, multipath characteristics
are identified for the plurality subscriber transmitted signals and these
characteristics are to combine multipath components of the plurality of
subscriber transmitted signals to produce a plurality of corrected
subscriber transmitted signals. Information related to a first signal is
extracted from the corrected subscriber transmitted signals and the first
signal is reconstructed, as originally received, where the reconstructed
first signal contains multipath scattering components of the first signal
as originally transmitted. Finally the reconstructed signal is inverse
summed to produce a third spread-spectrum signal substantially free of any
interference contributed by the first signal.
A final embodiment of the present invention encompasses an apparatus for
canceling interference in a spread-spectrum communication system
comprising a despreader having as an input a composite signal and
outputting a plurality of despread signals, a multipath identifier having
as an input the plurality of despread signals, and outputting multipath
characteristics of the plurality of despread signals, a RAKE finger
combiner having as an input the plurality of despread signals and the
multipath characteristics of the plurality of despread signals, and
outputting a first signal which is a representative of the composite
signal without multipath scattering components, a data decoder having as
an input, the first signal and outputting information related to an
individual signal, a signal reconstructor having as inputs, information
related to the individual signal and the multipath components, and
outputting the individual signal as originally received, wherein the
individual signal as originally received contains multipath scattering
components, and an inverse summer having as inputs the individual signal
as originally received and the composite signal and outputting the
composite signal substantially free of any interference contributed by the
individual signal.
FIG. 1 generally depicts, in block diagram form, receiver 100 which may
beneficially implement interference cancellation in accordance with the
invention. In a preferred embodiment of the present invention receiver 100
is contained within a cellular base station (not shown) such as a Motorola
SC9600 CDMA base station. Receiver 100 comprises downconverter 103,
oscillator 116, ordering generator 142, and a plurality of signal
canceling units 121. In the preferred embodiment of the present invention,
signal canceling units 121 comprise delay circuit 126, rake based signal
generator 122, and summer 123. Operation of receiver 100 in accordance
with a preferred embodiment of the present invention occurs as follows:
Uplink communication signals from multiple remote units (subscribers) are
received at downconverter 103. Receiver 100 determines or knows from
previously-stored information in receiver 100 the carrier phase, PN
spreading code, and data for each remote unit. In other words, receiver
100 contains knowledge of each of the received signals (SIGNAL.sub.1,
SIGNAL.sub.2, . . . , SIGNAL.sub.N) and thus cancellation of each of the
received signals from a particular received composite signal can be
achieved.
Continuing, in order to simplify hardware of the receiver 100, the
composite received signal is down converted to composite signal 120 at a
frequency of about 10 MHz by oscillator 116. Spread-spectrum composite
signal 120 is then input into canceling unit 121. As previously stated,
spread-spectrum composite signal 120 has undergone multipath scattering,
and as a result, spread-spectrum composite signal 120 comprises multiple
echoes for each subscriber. Canceling unit 121 splits composite signal 120
and inputs composite signal 120 into delay circuit 126 and RAKE-based
signal generator 122. Output from the RAKE-based signal generator 122 is
cancellation signal 124 and digital data 127, the generation of which is
described below. Cancellation signal 124 is then subtracted, via an
inverse summing node 128, with spread-spectrum composite signal 120 so
that any interference contributed by a chosen subscriber signal (e.g.,
SIGNAL.sub.1) is substantially eliminated. Resulting signal 130 represents
spread-spectrum composite signal 120 "clean" of any interference
contributed by the chosen subscriber signal. In the preferred embodiment
of the present invention, output signal 130 is then input into a second
canceling unit 121 to undergo substantially the same signal cancellation
procedure, except that subsequent processing by canceling units 121 will
remove interference contributed by other transmitted subscriber signals
(e.g., SIGNAL.sub.2 through SIGNAL.sub.N-1). Unlike prior-art methods of
signal cancellation, in the preferred embodiment of the present invention
signal generator 122 utilizes multipath scattering components of
SIGNAL.sub.N in the production of canceling signal 124. By taking
multipath scattering into consideration when synthesizing a replica of a
particular subscriber's received signal, and utilizing the synthesized
replica to cancel interference of a particular subscriber, the particular
subscriber's interference can be better removed from received composite
signal 120 than with prior-art techniques. Thus, the decoding of other
subscriber's signals with greater accuracy is thereby made possible using
the "subsequent" composite received signal (i.e., after interference
cancellation) without the contribution of the first subscriber.
FIG. 2 generally depicts, in block diagram form, RAKE based signal
generator 122 of FIG. 1 in accordance with the preferred embodiment of the
present invention. RAKE based signal generator 122 comprises despreader
201, multipath identifier 203, rake finger combiner 205, data decoder 207,
and signal reconstructor 209. Operation of RAKE based signal generator 122
occurs as follows: Composite signal 120 enters despreader 201. As
mentioned above, composite signal 120 comprises a multiplicity of
frequency and time overlapping coded (spread) signals each of which has
undergone multipath scattering. Despreader 201 despreads composite signal
120 to form signal 202 comprising a multiplicity of despread QPSK signals
representative of SIGNAL.sub.1 through SIGNAL.sub.N. In the preferred
embodiment of the present invention signal 202 is formed by despreading
composite signal 120 with the appropriate despreading code (PN Code) to
strip the spreading code from composite signal 120. The appropriate
despreading code is supplied to despreader by ordering generator 142
through input signal 160.
Signal 202 is then input into multipath identifier 203. Multipath
identifier 203 determines multipath characteristics for SIGNAL.sub.1
through SIGNAL.sub.N, which arise from the correlation peaks of the
various echoes. These multipath characteristics include, but are not
limited to, time delays and respective amplitudes and phases between
correlation peaks for each signal. For a general background on
identification of multipath components in communication systems, reference
is made to "Introduction to Spread-Spectrum Antimultipath Techniques and
Their Application to Urban Digital Radio" by Turin, published in the
Proceedings of the IEEE, Vol. 68, No. 3, March 1980. Multipath
characteristics 204 are output from multipath identifier 203 (along with
signal 202) and enter RAKE finger combiner 205. RAKE finger combiner 205
utilizes multipath characteristics 204 to combine multipath components of
SIGNAL.sub.1 through SIGNAL.sub.N resulting in signal 206 which is a
representation of signal 202 with "echoes" caused by multipath scattering.
Signal 206 is output to data decoder 207 which extracts information
related to a particular signal (e.g. SIGNAL.sub.1) from signal 206 and
outputs this information as resulting signal 208. In other words, data
decoder 207 receives signal 206, extracts information related to only one
signal (in this case SIGNAL.sub.1) and outputs this information as
resulting signal 208. In the preferred embodiment of the present invention
ordering generator 142 supplies data decoder 207 with information
regarding which signal to extract from signal 206. (Determination of which
signal to extract from signal 206 will be discussed below).
Continuing, signal reconstructor 209 receives signal 208, along with
multipath characteristics 204 (supplied by multipath identifier 203) and
"reconstructs" SIGNAL.sub.1 as originally received. (i.e., with echoes).
In other words, signal reconstructor 209 recreates the "echoes" that
originally existed in SIGNAL.sub.1 and outputs the reconstructed signal as
cancellation signal 124. Since cancellation signal 124 contains the
multipath scattering components of the transmitted signal (SIGNAL.sub.1),
prior-art interference cancellation techniques can be improved since a
better replica of the transmitted signal will be removed from composite
signal 120.
In the preferred embodiment of the present invention the accuracy of
cancellation signal 124 is improved by utilizing cancellation signals
representative of the most reliable transmitted signals (SIGNAL.sub.1,
SIGNAL.sub.2, . . . , SIGNAL.sub.N) prior to using cancellation signals
from less reliable signals. In other words, RAKE based signal generator
122 (existing within the first canceling unit 121 of FIG. 1) will utilize
the most reliable signal, while RAKE based signal generator 125 (existing
within the second canceling unit 121 of FIG. 1) will utilize the second
most reliable signal. (Note that to the second canceler the input signal
is clean from subscriber SIGNAL.sub.1). In order to determine the order of
cancellation, composite signal 120 is input into ordering generator 142.
Ordering generator 142 despreads composite signal 120 and rank orders each
signal (SIGNAL.sub.1, SIGNAL.sub.2, . . . , SIGNAL.sub.N) by received
signal strength. In the preferred embodiment of the present invention,
ordering generator 142 rank orders each signal by bit energy per noise
density (i.e., E.sub.b /N.sub.0, which is defined as the ratio of energy
per information-bit to noise-spectral density) associated with each
received signal. Ordering generator 142 outputs the appropriate signal to
decode to each RAKE based signal generator within receiver 100.
FIG. 3 is a flow chart illustrating operation of a signal cancellation unit
of FIG. 1. The logic flow begins at step 301 where despreader 201
despreads composite signal 120 to form signal 202 comprising a
multiplicity of despread QPSK signals representative of SIGNAL.sub.1
through SIGNAL.sub.N. Next, at step 303 multipath identifier 203
determines multipath characteristics for SIGNAL.sub.1 through
SIGNAL.sub.N. At step 305, RAKE finger combiner 205 utilizes multipath
characteristics 204 to combine multipath components of SIGNAL.sub.1
through SIGNAL.sub.N, resulting in signal 206 which is a representation of
signal 202 free from "echoes" caused by multipath scattering. At step 307
decoder 207 extracts information related to a particular signal (e.g.
SIGNAL.sub.1) from signal 206 and outputs this information as resulting
signal 208. Signal reconstructor 209 receives signal 208, along with
multipath characteristics 204 (supplied by multipath identifier 203) and
"reconstructs" SIGNAL.sub.1 as originally received. (step 309). Finally,
at step 311, the reconstructed signal is output to summing node 128 where
it is combined with delayed composite signal 120.
FIG. 4, generally depicts, in block diagram form, canceling unit 401 which
may beneficially implement interference cancellation in accordance with an
alternate embodiment of the present invention. Unlike the preferred
embodiment, in the alternate embodiment of the present invention canceling
units within receiver 400 comprise a most reliable signal selector 403
which determines the most reliable signal to decode and subtract from
composite signal 120. Canceling unit 401 comprises most reliable signal
selector 403, decoder 405, signal reconstructor 407, and summing circuit
409. Operation of canceling unit 401 occurs as follows: Uplink
communication signals from multiple remote units are received at
downconverter 103 and are downconverted to form composite signal 120.
Canceling unit 401 determines or knows from previously-stored information
in receiver 100, the carrier phase, PN spreading code, and data for each
remote unit. In other words, canceling unit 401 contains knowledge of each
of the received signals (SIGNAL.sub.1, SIGNAL.sub.2, . . . , SIGNAL.sub.N)
and thus cancellation of each of the received signals from a particular
received signal can be achieved.
Spread-spectrum composite signal 120 is then input into canceling unit 401
where it enters most reliable signal selector 403. Most reliable signal
selector 403 despreads composite signal 120, identifies multipath
components of composite signal 120, combines multipath components of
composite signal 120, and determines a most reliable signal (e.g.,
SIGNAL.sub.1) of composite signal 120. Most reliable signal selector has
three outputs: 1) multipath characteristics regarding composite signal
120, 2) signal 412 which is a representation of signal 120 free from
"echoes" caused by multipath scattering, and 3) identification of the most
reliable signal.
Signal 412 and Information on the most reliable signal are input into
decoder 405. Decoder 405 utilizes the identification of the most reliable
signal, extracts the most reliable signal (in this case SIGNAL.sub.1 from
signal 412 forming signal 414. Signal 414 is output to signal
reconstructor 407. Reconstructor 407 receives signal 414, along with
multipath characteristics (supplied by most reliable signal selector 403)
and "reconstructs" SIGNAL.sub.1 as originally received. (i.e., with
echoes). In other words, signal reconstructor 407 recreates the "echoes"
that originally existed in SIGNAL.sub.1 and outputs the reconstructed
signal as cancellation signal 424. Cancellation signal 424 is then
subtracted, via a inverse summing node 409, with spread-spectrum composite
signal 120 so that any interference contributed by a chosen signal (e.g.,
SIGNAL.sub.1 ) is substantially eliminated. Resulting signal 130
represents spread-spectrum composite signal 120 "clean" of any
interference contributed by the chosen signal. In the preferred embodiment
of the present invention, output signal 130 is then input into a second
canceling unit 440 to undergo substantially the same signal cancellation
procedure, except that subsequent processing by canceling units will
remove interference contributed by other transmitted signals (e.g.,
SIGNAL.sub.2 through SIGNAL.sub.N-1). Because cancellation signal 424
contains the multipath scattering components of the transmitted signal
(SIGNAL.sub.1, the particular subscriber's interference can be better
removed from received composite signal 120 than with prior-art techniques.
Thus, the decoding of other subscriber's signals with greater accuracy is
thereby made possible using the "subsequent" composite received signal
(i.e., after interference cancellation) without the contribution of the
first subscriber (i.e., SIGNAL.sub.1).
FIG. 5 generally depicts, in block diagram form, most reliable signal
selector 403 of FIG. 4 in accordance with the alternate embodiment of the
present invention. Signal selector 403 comprises despreader 501, multipath
identifier 510, RAKE finger combiner 515, and strongest signal selector
520. Operation of selector 403 in accordance with the alternate embodiment
of the present invention occurs as follows: Composite signal 120 enters
despreader 501. As mentioned above, composite signal 120 comprises a
multiplicity of frequency and time overlapping coded (spread) signals each
of which has undergone multipath scattering. Despreader 501 despreads
composite signal 120 to form signal 502 comprising a multiplicity of
despread QPSK signals representative of SIGNAL.sub.1 through SIGNAL.sub.N.
In the preferred embodiment of the present invention signal 502 is formed
by despreading composite signal 120 with the appropriate despreading code
(PN Code) to strip the spreading code from composite signal 120.
Signal 502 is then input into multipath identifier 510. Multipath
identifier 510 determines multipath characteristics for SIGNAL.sub.1
through SIGNAL.sub.N, which arise from the correlation peaks of the
various echoes. These multipath characteristics include, but are not
limited to, time delays and respective amplitudes and phases between
correlation peaks for each signal. Multipath characteristics are output
from multipath identifier 510 (along with signal 502) and enter RAKE
finger combiner 515. RAKE finger combiner 515 utilizes multipath
characteristics output from multipath identifier 510 to combine multipath
components of SIGNAL.sub.1 through SIGNAL.sub.N, resulting in signal 506
which is a representation of signal 502 free from "echoes" caused by
multipath scattering. Signal 506 is input into strongest signal selector
520, where the strongest signal (e.g., SIGNAL.sub.1) is determined by rank
ordering each signal by the E.sub.b /N.sub.0, associated with each
received signal. Selector 520 outputs the appropriate signal to decode,
(utilized by decoder 405).
While the invention has been particularly shown and described with
reference to a particular embodiment, it will be understood by those
skilled in the art that various changes in form and details may be made
therein without departing from the spirit and scope of the invention. For
example, in certain CDMA systems, such as IS-95, it is known that a pilot
signal is transmitted with all other user signals. This pilot signal
experiences the same multipath as each of the user signals transmitted by
that base station for a particular user. Therefore, RAKE analysis of the
pilot, and determination of the multipath parameters of the pilot obtained
by the multipath identification of the pilot signal, may be beneficially
used for multipath signal generation for each of the user's signals after
data decoding is accomplished at each stage. It is the intent of the
inventors that various modifications come within the scope of the
following claims.
Top